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Chromitite segregations in dunites of the Uktus Uralian-Alaskan-type complex (Central Urals, Russia) display large variation of the chromite composition: Cr/ (Cr+Al)=0.46-0.77, Fe 2+ /(Fe 2+ +Mg)=0.28-0.66, and Fe 3+ /(Fe 3+ +Fe 2+ )=0.23-0.59. Three types of PGM assemblages have been recognized, varying in accordance with chromite composition: type I, dominated by Ru-Os-Ir (sulfides), is associated with magnesiochromite having Fe 3+ /(Fe 3+ +Fe 2+ )<0.30, in the southern dunite body. Type II, containing abundant PtIr (alloys, minor sulfides), is found in magnesiochromite with Fe 3+ /(Fe 3+ +Fe 2+ )=0.40-0.44; type III, consisting of Ir-Rh-Pt-Pd (alloys, sulfarsenides, antimonides) in Fe-rich chromite having Fe 2+ /(Fe 2+ +Mg)=0.66 and Fe 3+ /(Fe 3+ +Fe 2+ )=0.59. Positive anomalies of Ir and Pt, and a negative peak of Ru characterize the PGE patterns of chromitites with type II and III PGM assemblages, whereas a positive Pt anomaly is observed in their dunite host. Intensive fractionation of Pt-Fe alloys in the Uktus chromitites reflects the anomalous behavior of Pt which is decoupled from Rh and Pd. Among other factors, the high iron activity and oxygen fugacity in the parent melt appear to exert a major control on precipitation of Pt-Fe alloys, below sulfur saturation. The strong Pt anomaly in chromitites from Uktus may indicate that Uralian-Alaskan-type magmas were derived from a Pt-rich mantle source.

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Chromitites from the Kytlym and Uktus zoned Uralian-Alaskan type complexes in the northern and central Urals, Russia, contain abundant inclusions of Pt alloys compositionally attributable to one of three main groups: 1) Pt-Fe alloys with an isoferroplatinum-type composition, Pt 3 Fe; 2) Ni-rich Pt-Fe-Ni-(Cu) alloys with a stoichiometry of the type isoferroplatinum Pt 2.5 (Fe,Ni,Cu) 1.5 , tetraferroplatinum Pt(Fe,Ni,Cu), and ferronickelplatinum Pt 2 FeNi, and 3) Cu-rich Pt-Fe-Cu-(Ni) alloys with a stoichiometry of tulameenite type, Pt 2 Fe(Cu,Ni). The Pt-Fe and Ni-rich Pt-Fe-Ni-(Cu) alloys occur as primary inclusions in unaltered chromian spinel and are considered to have formed in a high-temperature pre-chromite stage. A sulfide-rich assemblage (erlichmanite, Rh-Ir-Pt thiospinels, cooperite, unknown Rh-Ir-bearing base-metal sulfides, pyrrhotite and pentlandite) accompanies the Pt 3 Fe alloy, indicating that it was a stable phase under relatively high sulfur fugacity. The Pt-Fe-Ni-(Cu) alloys formed at low sulfur fugacity, as suggested by the absence of sulfides and the exclusive presence of native osmium in the assemblage. The Cu-rich Pt-Fe-Cu-(Ni) alloys of tulameenite type are exclusively located along cracks in contact with secondary ferrian chromite, magnetite and chlorite, or constitute the metasomatic replacement of primary Pt alloys, and are considered to have a low-temperature hydrothermal origin. The secondary assemblage of PGM also includes potarite, prassoite, rhodian pentlandite, Cu-Pd alloys, native osmium, Ir-Fe alloys and oxides. These data strongly suggest that Ural-Alaskan-type chromitites are major contributors of Pt alloys that are encountered associated with nuggets in placer deposits of the Ural Platinum-bearing Belt.

This paper reviews a database of about 1500 published and 1000 unpublished microprobe analyses of platinum-group minerals (PGM) from chromite deposits associated with ophiolites and Alaskan-type complexes of the Urals. Composition, texture, and paragenesis of unaltered PGM enclosed in fresh chromitite of the ophiolites indicate that the PGM formed by a sequence of crystallization events before, during, and probably after primary chromite precipitation. The most important controlling factors are sulfur fugacity and temperature. Laurite and Os–Ir–Ru alloys are pristine liquidus phases crystallized at high temperature and low sulfur fugacity: they were trapped in the chromite as solid particles. Oxygen thermobarometry supports that several chromitites underwent compositional equilibration down to 700 °C involving increase of the Fe3/Fe2 ratio. These chromitites contain a great number of PGM including—besides laurite and alloys—erlichmanite, Ir–Ni–sulfides, and Ir–Ru sulfarsenides formed by increasing sulfur fugacity. Correlation with chromite composition suggests that the latest stage of PGM crystallization might have occurred in the subsolidus. If platinum-group elements (PGE) were still present in solid chromite as dispersed atomic clusters, they could easily convert into discrete PGM inclusions splitting off the chromite during its re-crystallization under slow cooling-rate. The presence of primary PGM inclusions in fresh chromitite of the Alaskan-type complexes is restricted to ore bodies crystallized in equilibrium with the host dunite. The predominance of Pt–Fe alloys over sulfides is a strong indication for low sulfur fugacity, thereby early crystallization of laurite is observed only in one deposit. In most cases, Pt–Fe alloys crystallized and were trapped in chromite between 1300 and 1050 °C. On-cooling equilibration to ~900 °C may produce lamellar unmixing of different Pt–Fe phases and osmium. Precipitation of the Pt–Fe alloys locally is followed by an increase of sulfur fugacity leading to crystallize erlichmanite and Ir–Rh–Ni–Cu sulfides, occurring as epitaxic overgrowth on the alloy. There is evidence that the system moved quickly into the stabilization field of Pt–Fe alloys by an increase of the oxygen fugacity marked by an increase of the magnetite component in the chromite. In summary, the data support that most of the primary PGM inclusions in the chromitites of the Urals formed in situ, as part of the chromite precipitation event. However, in certain ophiolitic chromitites undergoing annealing conditions, there is evidence for subsolidus crystallization of discrete PGM from PGE atomic-clusters occurring in the chromite. This mechanism of formation does not require a true solid solution of PGE in the chromite structure.